Colloidal crystals built up by mono-dispersed particles have been known to yield aesthetic effects as a result of the periodic arrangement of the mono-dispersed particles therein. Specifically, a plurality of mono-dispersed particles is assembled in a closely-packed and regularly-ordered structure to form a layer of colloidal crystals. This highly organized structure, with the size of the mono-dispersed particles in the range of wavelength of visible light, selectively diffracts certain wavelengths and therefore renders a color corresponding to the diffracted wavelengths. The aesthetic effects can be optimized by adjusting the refractive index in the structure, changing materials or particle sizes of the mono-dispersed particles, etc.
The colloidal crystals layer may be applied to the external surface of any number of different substrates to provide visual effects to different articles, such as products and/or packaging. However, such articles may be subjected to different forms of wear and tear that could damage the photonic crystal structures provided on their external surfaces. If the photonic crystal structure is disrupted through such wear and tear, the visual effects provided by the photonic crystals may be impaired.
US 2014/0193649 describes thermal printing of thermally printable photonic crystal materials as a means of providing security measures on articles. The process of thermally printing causes some disruption to photonic crystals printed on a substrate such that it is necessary to apply a coating to the photonic crystals to protect them from abrasive damage. The coating of US 2014/0193649 is chosen to have a relatively high thermal transition such that it can protect the photonic crystals while being subjected to high temperatures. However, the coating of US 2014/0193649 does not protect against other damage or displacement of the photonic crystals. For this, a fixing coating is used to improve the permanency of the photonic crystal structure and to avoid disruption of the photonic crystal structure when exposed to e.g., thermal stimuli. The fixing coating may include monomer, prepolymer and/or polymer, a cross-linking agent, and a polymerization initiator. However, while the fixing coating retains the formation of the photonic crystal, there is no mention of ways of ensuring the photonic crystal structures stay properly adhered to the substrate to which they are applied.
Thus, there is a need to provide a method for improving the endurance of photonic crystals when applied to different substrates.